Acoustic touch position sensors are known to include a touch panel or plate having an array of transmitters positioned along a first edge of a substrate for simultaneously generating parallel surface acoustic waves that directionally propagate through the panel to a corresponding array of detectors positioned opposite the first array on a second edge of the substrate. Another pair of arrays is provided at right angles to the first set. Touching the panel at a point causes an attenuation of the waves passing through the point of touch, thus allowing an output from the two sets of arrays indicative of the coordinates of the touch. This type of acoustic touch position sensor is shown in U.S. Pat. No. 3,673,327.
The substrate in many embodiments is preferred to be transparent because this allows efficient and effective use of the touch sensor as a panel placed in front of a visual display device, such as a cathode ray tube, electroluminescent display, or liquid crystal display.
Acoustic touch position sensors are also known wherein a single transducer per axis is provided, which produces a surface acoustic wave which is reflected, by a reflective grating having elements set at 45.degree. to the beam, at right angles over the length of the grating to produce a surface acoustic wave pattern propagating through an active area of the substrate. The position of a touch in the active area is determined by, e.g., providing an opposing reflective grating which directs the surface acoustic wave pattern along an axis of the grating toward a receiving transducer system, which records the time of arrival of an attenuation of the wave pattern, which corresponds to a position along the axis of the arrays. The touch, in this case, may include a finger or stylus pressing against the surface. Other types of configurations for collecting the sensing signal are also known.
The reflective array is formed of acoustically partially reflective structure, which may be an inscribed or raised surface feature, or a feature having differing wave propagation characteristics which forms a partial barrier. These structural elements may, in theory, be formed at any portion where there is a significant wave energy. Thus, if a wave has surface energy, surface features may be used. If wave energy is buried, then these barriers must intrude into the material of the substrate. Thus, waves having surface energy, these reflecting arrays may be formed on the surface, and where wave energy is present on both sides of the substrate, these reflecting arrays may be formed on one or both sides of the substrate. Because the touch sensor is generally placed in front of a display device, and the reflective array is not generally optically invisible, the reflective arrays are generally placed at the periphery of the substrate, outside of the active sensing area, and are hidden or protected under a bezel.
There are a number of different types of acoustic wave propagation types which are supported by solid non-piezoelectric substrates, such as glass. Those having vertical, e.g., in the thickness direction, and longitudinal, e.g., in the direction of wave motion, components are referred to herein as VLCW, a type of surface acoustic wave. These VLCW, in substrates having a finite thickness, are either quasi-Rayleigh waves, which have surface energy primarily confined to a single surface over finite distances, or Lamb waves, which have surface energy on the front and rear surfaces of a substrate. Over extended distances, it becomes apparent that real quasi-Raleigh waves include a symmetric Lamb wave component and an antisymmetric Lamb wave component, and that these components have differing phase velocities. Thus, in realizable systems, surface waves are not truly confined to a single surface over large distances, and may lead to artifacts. These are discussed more fully below.
While a touch sensor employing quasi-Rayleigh waves have high sensitivity, this mode of operation may lead to high or even excessive sensitivity to contaminants or other materials abutting the active surface of the touch panel. The excessive sensitivity to contamination is due, in part, to the vertical component of wave motion at or near the confining surface. As a result, a large portion of the quasi-Rayleigh wave energy may be absorbed by even modest amounts of surface contaminants. The effect of near or total absorption of wave energy by contamination, sealants or other materials abutting the plate, is to create acoustic shadows or blind spots extending along the axes that intersect the contaminant. Thus, while quasi-Rayleigh mode touch sensors have a high sensitivity to touch, these sensors are also subject to shadowing in the event of a substantial contaminant in the active area, preventing localization of a touch on one or both axes of the sensor. A touch position sensor using quasi-Rayleigh waves therefore cannot localize touch if one or both coordinates is on a blinded axis.
Shear waves also propagate in substrates of finite thickness, and as employed in touch position sensors of the prior art, these shear waves are of zeroth order, meaning that there is no nodal plane intersecting the substrate, and the volumetric wave energy is uniform. Since shear waves do not have a vertical component of motion, small amounts of surface contaminants do not absorb all of the wave energy, and therefore do not completely shadow, and therefore shear mode sensors are more robust in contaminated environments. More complex waves, having multiple nodal planes intersecting a substrate are also known in theory, but these are generally considered complex interfering waves, without practical value for touch measurement or the like, and are therefore avoided for use in touch position sensors or eliminated if interfering. These complex waves have certain characteristics similar to zeroth order horizontally polarized shear waves, or more simply shear waves, yet other of their characteristics also differ markedly. These waves shall be herein referred to as higher order horizontally polarized shear-like waves, or HOHPS waves. Such waves have a high degree of dependence on the characteristics of the substrate and sensitivity to the configuration.
The wave pattern of known acoustic touch sensors is dispersed along the axis of the transmitting reflective array, traverses the substrate and is recombined, e.g., into an axially propagating wave, dispersed in time according to the path taken across the substrate, by another reflective grating, and is directed to a receiving transducer in a direction antiparallel to the transmitted wave, which receives the wave and converts it into an electrical signal for processing. Thus, according to this system, only two transducers per axis are required. Because of the antiparallel path, the time delay of a perturbation of the electrical signal corresponds to a distance travelled by the wave, which in turn is related to the axial distance from the transducer along the reflecting arrays travelled by the wave before entering the active area of the substrate. The location of touch is determined by detecting an attenuated signal as compared to a standard received waveform. Thus, for each axis, a distance may be determined, and with two orthogonal axes, a unique coordinate for the attenuation determined. Acoustic touch position sensors of this type are shown in U.S. Pat. Nos. 4,642,423, 4,644,100, 4,645,870, 4,700,176, 4,746,914 and 4,791,416, incorporated herein by reference.
For each axis, a standard signal is provided to the transducer by interfacing a piezoelectric transducer with the sheet-like member, outside the active area, to produce a wave, propagating along an axis. For example, surface waves are generally coupled through the surface portion of the substrate, on the side which is intended to be touch sensitive. The reflective array in the path of the wave, for redirecting the wave into the touch sensitive region, includes a series of spaced surface interruptions, having a separation of an integral number of wavelengths of the wave produced by the transmitting transducer, angled 45.degree. to the axis, i.e., the direction of wave propagation. The reflective array thus produces a reflected a surface acoustic wave propagating at 90.degree. to the original angle of transmission, through the active area of the substrate.
As shown in FIGS. 1A and 1C, surface acoustic waves may be transduced, with a certain coupling efficiency, into a touch plate utilizing a transducer mounted on a wedge that is in turn mounted on the touch surface of the plate, wherein the transducer vibrates in the direction shown to produce a compressional bulk wave that propagates in the wedge, which in turn is transduced through the wedge-substrate interface to impart a surface acoustic wave, i.e., a wave having vertically and longitudinal components (VLCW), in the touch plate. The wedge extends above the plate, and therefore the rear or inactive side of the substrate and its edges remain free of circuitry or critical elements. Further, the area of the substrate in which the wave energy is in the form of quasi-Rayleigh mode waves is insensitive to mounting on the opposite, inactive surface. Coupling wedges are typically made of plastic, and mounted to a glass plate. The transducer, which is generally a piezoelectric element with electrically conducting pads on two large area opposing faces, is bonded to the plastic wedge with a conductive element in between, and the transducer with wedge is then bonded to the glass touch plate substrate, with the sandwiched conductive element and opposing electrically conductive pad connected to the electrical circuitry.
To receive the sensing wave, it is generally considered desirable to provide a single transducer for transducing the wave into an electrical signal in which the touch position is encoded by temporal fluctuations in the signal. While a transducer the full length of the substrate could be provided, this requires a large transducer. Instead, the art teaches an inverse of the transmission technique, multiplexing the sensing wave into a surface wave directed toward a small receiving transducer. Thus, in an area outside the active area, the waves are again reflected by an otherwise identical reflecting array having spaced interruptions at -45.degree. to the angle of wave propagation, thereby multiplexing the spatially dispersed signal into a single waveform pattern, propagating antiparallel to the transmitted surface acoustic wave, which is detected by another transducer. In known systems, the excitation frequency is generally around 5 MHz or 5.5 MHz, and the thickness of the sheet-like member, if formed of soda-lime glass is typically in the range from 0.090" to 0.125".
The art also teaches the use of a single transducer for both transmitting the wave and receiving the sensing wave, with a single reflective array employed to disperse and recombine the wave. Such systems therefore employ a reflective edge opposite the reflective array. As a result, the SAW wave passes through the active region twice, with consequent increased wave absorption by the touch but also increased overall signal attenuation due to the reflection and additional pass through the active region of the substrate. Thus, the SAW wave may be reflected off an edge of the substrate parallel to the axis of the transmission reflective grating and reflected back through the substrate to the reflective array and retrace its path back to the transducer. The transducer, in this case, is configured to act as both transmitter and receiver at appropriate time periods. A second transducer, reflective array and reflective edge are provided for an axis at right angles to allow determination of a coordinate of touch.
A related system provides for a single transducer which produces a sensing wave for detecting touch on two axes, which both produces the acoustic wave and also receives the wave from both axes. In this case, the area in which touch is to be sensed is generally oblong, such that the longest characteristic delay along one path is shorter than the shortest characteristic delay along the second path.
Adler, U.S. Pat. Re No. 33,151, relates to a touch-sensitive system for determining a position of a touch along an axis on a surface. A SAW generator is coupled to a sheet-like substrate to generate a burst of waves, which are deflected into an active region of the system by an array of wave redirecting gratings. Quasi-Rayleigh waves traversing the active region are in turn redirected along an axis by gratings to an output transducer. The redirecting gratings are oriented at 45.degree. to the axis of propagation. A location of touch is determined by analyzing a selective attenuation of the received waveform in the time domain, each characteristic delay corresponding to a locus on the surface. The grating elements are placed at a 45.degree. angle and spaced at integral multiples of the quasi-Rayleigh wavelength with dropped elements to produce an approximately constant SAW power density over the active area. Thus the spacing between grates decreases with increasing distance along the axis of propagation from the transducer, with a minimum spacing of one wavelength of the transmitted wave. U.S. Pat. No. 4,746,916 also teaches use of reflecting elements which vary in height to control a ratio of reflected wave power to unreflected wave power.
Brenner et al., U.S. Pat. No. 4,644,100 relates to a touch sensitive system employing surface acoustic waves, responsive to both the location and magnitude of a perturbation of the SAW. The system according to U.S. Pat. No. 4,644,100 is similar in execution to the system according to U.S. Pat. Re. No. 33,151, while determining an amplitude of a received wave and comparing it to a stored reference profile.
Optimization of the performance of a touch sensor using Rayleigh waves is difficult because touch sensitivity and minimum touch panel thickness are not independent choices. In order to support a quasi-Rayleigh wave in a touch panel of reduced thickness, its other dimensions remaining the same, the wavelength must be reduced to preserve single surface confinement over the active area of the substrate. It is characteristic of quasi-Rayleigh waves that their confinement depth is related to wavelength, with confinement depth decreasing as the wavelength is decreased. As a result, the wave is confined to a shallower region bounded by the surface, and the proportion of wave energy absorbed by a given absorbing medium at the surface is increased, approximately by the inverse square of the wavelength. Thus, quasi-Rayleigh wave touch sensors can be considered unduly sensitive for some applications, even for relatively thick panels, hence the effect of reducing touch panel thickness results in touch sensors even more sensitive to surface contamination and other abutments. Conversely, the prior art teaches that reducing sensitivity by increasing the quasi-Rayleigh wavelength results in increased panel thickness and weight.
Surface waves having vertical and longitudinal components, with substantial energy through the thickness of the substrate, are termed Lamb waves, and systems incorporating same to detect touch position are disclosed in U.S. Pat. No. 5,072,427 and U.S. Pat. No. 5,162,618. As discussed above, Lamb waves are dispersive, varying in phase and velocity, so that various Lamb mode waves propagating in a substrate will interfere with one another. Therefore, systems employing Lamb waves also include elements to separate or eliminate unwanted or interfering propagation modes.
For example, it is known to place elements of the reflective arrays on both sides of the substrate in order to facilitate selection between the zeroth order symmetric and zeroth order anti-symmetric wave, by distinguishing between the phase of the array, i.e., placement and spacing of the elements, on the upper and lower surfaces of the substrate.
Acoustic touch position sensors utilizing surface acoustic waves as taught by the above-mentioned patents have a number of problems which are more readily understood when the nature of the surface acoustic wave used in these sensors is more fully considered. If, as in the above mentioned patents, the touch plate consists of a uniform, non-piezoelectric medium, and the acoustic wave is confined at or near a single surface such as an outer surface of the touch plate, the surface acoustic wave is known as a Rayleigh wave. These waves have X and Z components such that disturbed particles move elliptically in the X-Z plane. It is characteristic of these waves that the volumetric wave energy rapidly decreases with depth, so that the wave energy along the Z axis is essentially confined at or near the surface of the touch plate.
Theoretically, true Rayleigh waves exist only in an infinitely thick medium. Waves in a uniform, non-piezoelectric medium of finite thickness that are essentially confined to a single surface, as shown in FIGS. 1A-1D, are more precisely termed quasi-Rayleigh waves. Given a long enough propagating path in a medium of finite thickness, Rayleigh-type wave energy will not be confined at or near a single surface, but will transfer back and forth between the outer surfaces of the plate. This is because, although small, wave energy components extend through the material thickness, consisting of at least a symmetric and an antisymmetric waveform, each of which travels at a slightly different phase velocity, thus causing constructive and destructive interference at various spaced loci on the surfaces of the substrate, i.e., loci on the surfaces which have constructive or destructive interference of the two wave components. Areas of low surface wave energy are insensitive to perturbing influences, e.g., touch, while areas of relatively high surface energy may be overly sensitive and contribute to artifacts.
If the thickness of a touch plate substrate is, for example, two Rayleigh wavelengths or less, the waves emanating from the source transducers are Lamb mode, and are clearly distinguishable from quasi-Rayleigh mode, as well as other surface acoustic wave (SAW) modes, shown in FIGS. 1E and 1F. Lamb waves exist in two groups of various orders, each of which propagates independently of one another. One group is characterized by particle displacement that is symmetric with respect to the median plane of the plate. The other group of Lamb waves is characterized by particle displacement that is anti-symmetric with respect to the median plane. In general, a specific order within the symmetric Lamb wave group differs in phase and group velocity from the identical order of the anti-symmetric Lamb wave group. In particular, with a sufficient plate thickness equal to or greater than two Rayleigh wavelengths, two modes of approximately equal amplitude are principally excited, the zeroth order symmetrical Lamb waves and the zeroth order anti-symmetrical Lamb waves. As seen in FIGS. 1E and 1F, the symmetrical and anti-symmetrical Lamb waves are not confined to a single surface of the touch plate, but extend through the plate to the opposite surface thereof. When "in phase" however, such as the condition that is initially at and close to the source of the waves, where the waves constructively interfere on one surface and destructively interfere on the other surface, the two Lamb waves combine to produce a quasi-Rayleigh wave, as can be seen from a comparison of FIGS. 1E and 1F to FIG. 1D. As the two Lamb wave modes travel further from the source, due to the differing phase velocities and the resultant phase difference between them, there is an apparent complete transference of wave energy from the outer surface on which the transducer generating the wave is mounted, to the opposite surface. This transference of energy between the outer surfaces of the plate occurs at regularly spaced intervals, making a touch plate having large enough dimensions for this transference to occur, generally unsuitable for a touch position sensor, so long as both wave modes are present. As proposed in the above patents, these modes may be filtered and the unwanted modes eliminated.
In the case of a substrate of about four Rayleigh wavelengths thick, most of the wave energy of a quasi-Rayleigh wave remains on a single surface, and therefore a substrate of limited size is sensitive without dead spots due to wave dispersion over its entire surface. Thus, touch position sensors utilizing surface acoustic waves, and more particularly quasi-Rayleigh waves, operate with relatively thick panels, i.e., panels having a thickness of three to four times the wavelength of the surface acoustic wave propagating therein, with the quasi-Rayleigh waves confined at and near a single surface.
A touch sensor according to the above Rayleigh mode wave touchsensor patents, would be inoperable with a thin glass substrate, e.g., 0.045" soda-lime glass with an operating frequency of about 5 MHz, because a touch in a region of one outer surface where complete transference of the wave energy to the opposite outer surface has taken place, will not detectably disturb the wave, and is therefore limits the usability of the sensor for the intended purpose. In practice, in order to provide a wave that is practically confined to a single surface, the thickness of the touch plate must be at least three to four times the wavelength of the wave imparted into the substrate, wherein the length and breadth of the touch plate are also limited. The substrate may have dimension from about 25 to in excess of 1000 wavelengths of the SAW wave, which allows substrates of about 1.5 to 70 cm with an operating wavelength of 0.0574 cm, and operating frequency of 5.53 MHz.
It is thus known to use, instead of a quasi-Raleigh wave, a order Lamb wave transmitted through the bulk sheet-like material. Such a wave travels through the full thickness of the substrate, and the system is sensitive to touch on both surfaces of the sheet. This creates difficulty in mounting the substrate, which is generally a transparent substrate for placement in front of a cathode ray display (CRT) tube. Further, related to this sensitivity on both sheet surfaces, the signal-to-noise ratio is reduced, due to wave energy on both an active and inactive surface. In addition, vertical components of transverse wave propagation are highly sensitive to attenuation by small amounts of water, which may create artifacts.
In a zeroth order Lamb mode system, the maximum thickness of the substrate is limited to about two times the wavelength of the excited wave. Thicker substrates, under similar conditions, would propagate such waves as quasi-Raleigh mode waves, and substantial energy of the wave would not be present through the full thickness of the substrate. Thin glass has a greater difference in phase velocity of the symmetric and antisymmetric Lamb mode waves propagating therein than a thicker substrate, which allows simplified and selective filtering of wave modes, as suggested in U.S. Pat. No. 5,072,427.
It is known to create a zeroth order Lamb mode wave from a zeroth order shear wave, by reflection of the shear wave by an array of parallel interruptions spaced along the axis of propagation of the shear wave at integral multiples of the wavelength of the desired Lamb wave mode in the substrate, the reflective elements being situated at an angle of: EQU .theta..sub.A =arctan (V.sub.L /V.sub.Z)
where .theta..sub.A is the angle of the converting elements to the X axis, V.sub.L is the phase velocity of the desired Lamb mode wave and V.sub.Z is the velocity of the zeroth order horizontally polarized shear wave propagating along the axis of the array.
Likewise, a propagating zeroth order horizontally polarized shear wave may be converted to a Lamb wave by arrays having elements on both surfaces of the substrate, which facilitates selection between the zeroth order symmetric and zeroth order antisymmetric Lamb wave, by the relative phase of the array on the upper and lower surfaces of the substrate. The Lamb mode wave is reflected by a reflecting array, with elements on one or both sides of the substrate, to an opposed receiving transducer system, or may be reflected off a distal edge of the substrate and transmitted back through the active area, where it is reflected along the axis of the array as a zeroth order horizontally polarized shear wave, for detection by a combination excitation and reception transducer.
It has also been proposed to impart zeroth order shear waves into a substrate which are converted into Lamb waves by a reflective array, which are then converted by a reflective array back into shear waves for detection of the zeroth order component, see U.S. Pat. No. 5,072,427.
Shear wave mode touch sensors are also known. These systems operate by exciting zeroth order shear waves, which are non-dispersive, in a substrate. A touch absorbs only a small percentage of the shear wave energy intercepted by a touched surface, thus leading to relatively low sensitivity but also improved resistance to shadowing artifacts. In fact, the percentage of intercepted energy absorbed by a given touch is about five times greater for a Rayleigh wave than it is for a comparable zeroth order horizontally polarized shear wave for practical touch plate thickness. Such a zeroth order horizontally polarized shear wave system is proposed in U.S. Pat. No. 5,177,327. See also Knowles, T. J., "46.6: A Pressure-Responsive Touch-input Device" SID 92 Digest, (1992) pp. 920-923; Christensen, R. and Masters, T., "Guided Acoustic Wave: Newest Wave in Touch Technology", ECN (January 1995), pp. 13 et seq.
Increasing the thickness of the substrate facilitates HOHPS wave propagation, which heretofore has been considered parasitic. Therefore, known shear wave touch sensors have expressly limited substrate thickness to help limit or eliminate these HOHPS modes, as well as other wave modes, such as quasi-Rayleigh waves. Higher order horizontally polarized shear waves are dispersive, and therefore a configuration which promotes wave energy components of these types has been considered difficult or unworkable. Thus, in theory, mixed HOHPS waves in the active region of the touch sensor would follow pattern similar to that exhibited by Lamb waves, having "dead spots" on the substrate surface due to dispersion of the various components.
Zeroth order horizontally polarized shear wave touch sensors may be arbitrarily thin, limited only by their structural integrity. It is known, in order to limit potentially interfering modes, that the thickness of the substrate in a zeroth order horizontally polarized shear wave touch position sensor be less than that capable of supporting quasi-Rayleigh waves, since the thinner the substrate, the greater the fractional sensitivity of the horizontally polarized shear wave to a surface touch.
According to U.S. Pat. No. 5,177,327, a thin glass substrate, e.g., 0.040" thick, is desired because it has higher sensitivity to touch-induced attenuation of zeroth order horizontally polarized shear waves in the active area, has a lower weight than thicker glass, e.g., 0.090", as is often used in quasi-Raleigh mode touch sensors. However, 0.090" glass is easier to form and less fragile than 0.040" thick glass.
Knowles, U.S. Pat. No. 5,329,070 relates to a zeroth order horizontally polarized shear wave touch localization sensor. U.S. Pat. No. 5,329,070 indicates that an advantage of using a zeroth order horizontally polarized shear wave is that the sensor may be arbitrarily thin, and in fact the substrate is maintained at a thickness of less than 2 wavelengths in order to suppress higher order modes or overtones. In order to increase the thickness of the substrate in excess of two wavelengths for superior rigidity, a back plate is bonded by an adhesive which does not support shear wave propagation, or the back plate has a shear wave propagation velocity greater than the substrate shear wave propagation velocity. The latter type of shear wave is known as a Love wave.
Zeroth order horizontally polarized shear waves are not confined to the surface of the substrate, as are quasi-Rayleigh waves, but rather extend throughout the entire thickness of the substrate. Contaminants or other materials abutting the surface of a shear wave touch position sensor are less likely to result in blind spots or significant shadows extending along the axes that intersect the contaminate or matter. VLCW have a vertical transverse component and are substantially absorbed by water droplets on the surface while shear and shear-type waves, without a vertical transverse component do not substantially radiate pressure waves into water droplets at the surface, and are thus do not have undesired sensitivity to these contaminants. Thus, a significant portion of the wave energy will pass through the location of a touch or surface artifact. Therefore, shear wave touch position sensors are suitable for use in environments that surface acoustic wave sensors may not be. Shear wave touch position sensors are also sensitive to a touch on both the top and bottom surfaces of the substrate whereas surface acoustic wave sensors are sensitive to touch only on the surface of the substrate on which the transducer is mounted. These factors, as well as wave properties allow comparable shear mode waves to travel over greater distances than surface acoustic wave.
Zeroth order horizontally polarized shear waves thus are known to have several advantages over surface acoustic waves, which compensate for the lower percentage of total energy absorbed by a touch. As known in the art, however, the proposed implementations are difficult to work with and present their own difficulties and limitations.